Atrial fibrillation (AF) is a well-known disorder of the heart, which causes hemodynamic efficiency to be reduced and, in serious cases, can lead to cardiac embolization, stroke, ventricular arrhythmias and other potentially fatal complications.
AF is frequently engendered by abnormal electrical conduction paths within the heart muscle. Normally, electrical activation signals are conducted in an orderly way through the atrium and into the ventricle, passing each point in the heart only once in each heart cycle. Electrical activation signals at different locations in the heart are well correlated, taking into account normal propagation delays from one region of the heart to another. In response to local activation signals, the atrial muscle fibers contract in proper synchrony, to pump blood through the atrium. In AF, however, this orderly contraction is lost, it is believed, as multiple, changing, spatially disorganized activation wavelets sweep across the surface of the atria, resulting in irregular patterns of electrical activation. A given atrial muscle fiber is activated to contract multiple times in each heart cycle, and fibrillation takes the place of normal contraction.
These phenomena are described in detail by Gregory W. Botteron and Joseph M. Smith in an article entitled, "A Technique for Measurement of the Extent of Spatial Organization of Atrial Activation During Atrial Fibrillation in the Intact Human Heart," in IEEE Transactions on Biomedical Engineering 12 (June 1995), pages 579-586, and in a second article entitled, "Quantitative Assessment of the Spatial Organization of Atrial Fibrillation in the Intact Human Heart," in Circulation 93 (Feb. 1, 1996), pages 513-518. Both of these articles are incorporated herein by reference.
FIG. 1 schematically illustrates abnormal activation paths as encountered in atrial tissue 10 of a heart undergoing AF, following the description of Botteron and Smith. Multiple wavelets 12 are generated by reentrant activation signals, following circuitous paths in tissue 10. Each wavelet dominates a corresponding one of a plurality of spatial domains 14, so that within a given domain, the activation signals will generally be highly mutually correlated, while there will be little or no correlation between signals in different domains. It will be understood that the boundaries between domains 14, shown as dashed lines in the figure, are generally not fixed. Rather, the paths of wavelets 12 and domains 14 typically vary over time.
The minimum size of any one of the domains 14 is generally controlled by the minimum circumference of the circle described by the corresponding wavelet 12, which is roughly equal to a tissue dimension value D (referred to by Botteron and Smith as the tissue wavelength), given by the product of the tissue's conduction velocity and its refractory period. Typically D is on the order of 18 mm.
It will be appreciated that electrical activation signals at two locations in mutual proximity, within the same domain 14, will generally be well correlated. This correlation has been found to drop off inversely, generally exponentially, as a function of distance, so that in conditions of AF, signals at more distant locations, in different domains, are poorly correlated. By comparison, under conditions of orderly conduction within the heart tissue, the electrical activation signals will be well-correlated over substantially the entire heart, taking into account normal conduction delays between one location in the heart and another.
Although drug therapy or implantation of a pacemaker is frequently useful in controlling AF, when these methods are unsuccessful, the preferred method of treatment of the condition is to invasively interrupt the abnormal conduction paths in the heart. Preferably, a catheter having an RF ablation electrode is passed percutaneously through a blood vessel into the atrium of the heart. The electrode at the catheter tip is brought into contact with one or more sites in the endocardium where an abnormal conduction path is believed to pass, and the electrode is activated to ablate the site(s) and, it is hoped, break the abnormal path(s).
Generally, however, it is difficult or impossible to know the precise abnormal conduction path. Furthermore, even if an abnormal path is broken at one site, other abnormal paths may exist or new paths may arise at other sites, which paths will cause the AF to continue even after the one or more sites in the endocardium are ablated. In response to this difficulty, some cardiologists and cardiac surgeons have used a "maze procedure," as described, for example, by T. Bruce Ferguson, Jr., and James L. Cox in an article entitled, "Surgery for Atrial Fibrillation," in Cardiac Electrophysiology: From Cell to Bedside, Second Edition, Douglas P. Zipes and Jose Jalife, eds., (W. B. Saunders Company, 1995), pages 1563-1576, which is incorporated herein by reference. In this procedure, multiple elongated strips in the endocardium are surgically cut or ablated in a direction generally parallel to the desired, normal, direction of conduction in the atrium. This procedure is time-consuming and causes far more damage to the endocardium than would be necessary if the abnormal paths could be selectively ablated.
U.S. Pat. No. 5,450,846, whose disclosure is incorporated herein by reference, describes a catheter, which may be repeatedly repositioned inside the heart, comprising an ablator at its distal tip and pairs of non-contacting sensing electrodes arrayed around the outside of the catheter near the distal end. Each electrode pair senses local electrogram signals generated in the endocardium in a small area near the side of the catheter that it faces. Differences in the activation times in the signals sensed by the pairs of electrodes are used to estimate the direction of the activation vector in the vicinity of the catheter, so as to guide the operator in positioning the ablator. This catheter is useful, however, only when electrical activation paths within the heart are relatively orderly, and not in the chaotic jumble of activation paths that generally characterizes AF.